Diabetes mellitus can be divided into two clinical syndromes, Type I and Type II diabetes mellitus. Type I diabetes, or insulin-dependent diabetes mellitus, is a chronic autoimmune disease characterized by the extensive loss of beta cells in the pancreatic islets of Langerhans (hereinafter referred to as “pancreatic islet cells” or “islet cells”), which produce insulin. As these cells are progressively destroyed, the amount of secreted insulin decreases, eventually leading to hyperglycemia (abnormally high level of glucose in the blood) when the amount secreted drops below the level required for euglycemia (normal blood glucose level). Although the exact trigger for this immune response is not known, patients with Type I diabetes have high levels of antibodies against pancreatic beta cells (hereinafter “beta cells”). However, not all patients with high levels of these antibodies develop Type I diabetes.
Type II diabetes, or non-insulin-dependent diabetes mellitus, develops when muscle, fat and liver cells fail to respond normally to insulin. This failure to respond (called insulin resistance) may be due to reduced numbers of insulin receptors on these cells, or a dysfunction of signaling pathways within the cells, or both. The beta cells initially compensate for this insulin resistance by increasing their insulin output. Over time, these cells become unable to produce enough insulin to maintain normal glucose levels, indicating progression to Type II diabetes (Kahn S E, Am. J. Med. (2000) 108 Suppl 6a, 2S-8S).
The fasting hyperglycemia that characterizes Type II diabetes occurs as a consequence of the combined lesions of insulin resistance and beta cell dysfunction. The beta cell defect has two components: the first component, an elevation of basal insulin release (occurring in the presence of low, non-stimulatory glucose concentrations), is observed in obese, insulin-resistant pre-diabetic stages as well as in Type II diabetes. The second component is a failure to increase insulin release above the already elevated basal output in response to a hyperglycemic challenge. This lesion is absent in pre-diabetes and appears to define the transition from normo-glycemic insulin-resistant states to frank diabetes. There is currently no cure for diabetes. Conventional treatments for diabetes are very limited, and focus on attempting to control blood glucose levels in order to minimize or delay complications. Current treatments target either insulin resistance (metformin, thiazolidinediones (“TZDs”)), or insulin release from the beta cell (sulphonylureas, exanatide). Sulphonylureas, and other compounds that act by depolarizing the beta cell, have the side effect of hypoglycemia since they cause insulin secretion independent of circulating glucose levels. One approved drug, Byetta (exanatide) stimulates insulin secretion only in the presence of high glucose, but is not orally available and must be injected. Januvia (sitagliptin) is another recently approved drug that increases blood levels of incretin hormones, which can increase insulin secretion, reduce glucagon secretion and have other less well characterized effects. However, Januvia and other dipeptidyl peptidases IV inhibitors may also influence the tissue levels of other hormones and peptides, and the long-term consequences of this broader effect have not been fully investigated. There is an unmet need for oral drugs that stimulate insulin secretion in a glucose dependent manner.
Progressive insulin resistance and loss of insulin secreting pancreatic β-cells are primary characteristics of Type II diabetes. Normally, a decline in the insulin sensitivity of muscle and fat is compensated for by increases in insulin secretion from the β-cell. However, loss of β-cell function and mass results in insulin insufficiency and diabetes (Kahn B B, Cell 92:593-596, 1998; Cavaghan M K, et al., J. Clin. Invest. 106:329-333. 2000; Saltiel A R, Cell 104:517-529, 2001; Prentki M and Nolan C J. J Clin Invest. 116:1802-1812. (2006); and Kahn S E. J. Clin. Endocrinol. Metab. 86:4047-4058, 2001). Hyperglycemia further accelerates the decline in β-cell function (UKPDS Group, J.A.M.A. 281:2005-2012, 1999; Levy J, et al., Diabetes Med. 15:290-296, 1998; and Zhou Y P, et al., J Biol Chem 278:51316-23, 2003). Several of the genes in which allelic variation is associated with an increased risk of Type II diabetes are expressed selectively in the beta cell (Bell G I and Polonsky K S, Nature 414:788-791 (2001); Saxena R, et al., Science. (2007) April 26; [Epub ahead of print]; and Valgerdur Steinthorsdottir, et al., Nature Genetics (2007) April 26; [Epub ahead of print]).
Insulin secretion from the beta cells of pancreatic islets is elicited by increased levels of blood glucose. Glucose is taken up into the beta cell primarily by the beta cell and liver selective transporter GLUT2 (Thorens B. Mol Membr Biol. 2001 October-December; 18(4):265-73). Once inside the cell, glucose is phosphorylated by glucokinase, which is the primary glucose sensor in the beta cell since it catalyzes the irreversible rate limiting step for glucose metabolism (Matschinsky F M. Curr Diab Rep. 2005 June; 5(3):171-6). The rate of glucose-6-phosphate production by glucokinase is dependent on the concentration of glucose around the beta cell, and therefore this enzyme allows for a direct relationship between level of glucose in the blood and the overall rate of glucose oxidation by the cell. Mutations in glucokinase produce abnormalities in glucose dependent insulin secretion in humans giving further evidence that this hexokinase family member plays a key role in the islet response to glucose (Gloyn A L, et al., J Biol. Chem. 2005 Apr. 8; 280(14):14105-13. Epub 2005 Jan. 25). Small molecule activators of glucokinase enhance insulin secretion and may provide a route for therapeutic exploitation of the role of this enzyme (Guertin K R and Grimsby J. Curr Med. Chem. 2006; 13(15):1839-43; and Matschinsky F M, et al., Diabetes 2006 January; 55(1):1-12) in diabetes. Glucose metabolism via glycolysis and mitochondrial oxidative phosphorylation ultimately results in ATP production, and the amount of ATP produced in a beta cell is directly related to the concentration of glucose to which the beta cell is exposed.
Elevated ratios of ATP to ADP that occur in the presence of higher glucose result in the closure of the Kir6.2 channel via interaction with the SUR1 subunit of the channel complex. Closure of these channels on the plasma membrane of the beta cell results in depolarization of the membrane and subsequent activation of voltage dependent calcium channels (VDCCs) (Ashcroft F M, and Gribble F M, Diabetologia 42:903-919, 1999; and Seino S, Anna Rev Physiol. 61:337-362, 1999). Calcium ion entry as well as release of calcium from intracellular stores triggers exocytosis of insulin granules, resulting is secretion of insulin into the blood stream. Agents which close the Kir6.2 channel such as sulphonylureas and metaglitinides (Rendell M. Drugs 2004; 64(12):1339-58; and Blickle J F, Diabetes Metab. 2006 April; 32(2):113-20) also cause membrane depolarization, and therefore these agents stimulate insulin secretion in a glucose independent fashion. Potassium channel openers, such as diazoxide, inhibit insulin secretion by preventing elevated ATP/ADP ratios from closing the Kir6.2 channel (Hansen J B. Curr Med. Chem. 2006; 13(4):361-76). Calcium channel blockers, such as verapamil and nifedipine, can also inhibit insulin secretion (Henquin, J. C. (2004) Diabetes 53, S48-S58). Although sulfonylureas and metaglitinides are effective glucose lowering agents in the clinic, they act independently of blood glucose levels. Because they act independently of glucose levels, these drugs may result in hypoglycemia.
Glucose dependent insulin secretion from the beta cell is dependent on numerous neurotransmitters and blood-borne hormones, as well as local, intra-islet factors. CNS activation of the vagal innervation of the islet can lead to the release of small molecules such as acetylcholine and peptides such as vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and Pituitary Adenylate Cyclase Activating Peptide (PACAP). Acetylcholine activation of phospholipase C through the Gαq-coupled GPCR M3 muscarinic receptor leads to release of Ca++ from intracellular stores (Gilon P, and Henquin J C. Endocr Rev. 2001 October; 22(5):565-604). Cholinergic agonists also lead to a subtle Na+-dependent plasma membrane depolarization that can work in concert with glucose-initiated depolarization to enhance insulin release (Gilon P, and Henquin J C. Endocr Rev. 2001 October; 22(5):565-604). VIP and PACAP each bind to an overlapping set of Gα-coupled GPCRs (PAC1, VIPR1, and VIPR2) on the beta cell that lead to stimulation of adenylate cyclase and an increase in intracellular cAMP (Filipsson K, et al., Diabetes, 2001 September; 50(9):1959-69; Yamada H, et al., Regal Pept. 2004 Dec. 15; 123(1-3):147-53; and Qader S S, et al., Am J Physiol Enclocrinol Metab. 2007 May; 292(5):E1447-55).
Elevation of beta cell cAMP has a substantial potentiating effect on insulin secretion in the presence of stimulatory levels of glucose (see below). Unfortunately, many potentiators of glucose-stimulated insulin secretion also have effects outside of the islet which limit their ability to be used as diabetes therapeutics. For example, the best available selective muscarinic agonists which stimulate insulin secretion also stimulate multiple undesirable responses in multiple tissues (Rhoades R A and Tanner G A, eds. (2003) Medical Physiology, 2nd ed. Lippincott, Williams and Wilkins. ISBN 0-7817-1936-4). Likewise, VIP and PACAP receptors are present in multiple organ systems and mediate effects on the reproductive, immune and other diverse systems that make them less attractive as specific enhancers of glucose dependent insulin secretion.
Incretin hormones such as Glucagon-Like Peptide 1 (GLP-1) and Glucose-dependent Insulinotropic Polypeptide (GIP, also known as Gastric Inhibitory Polypeptide) also bind to specific Galphas-coupled GPCRs receptors on the surface of islet cells, including beta cells, and raise intracellular cAMP (Drucker D J, J Clin Invest. 2007 January; 117(1):24-32). Although the receptors for these hormones are present in other cells and tissues, the overall sum of effects of these peptides appear to be beneficial to control of glucose metabolism in the organism (Hansotia T, et al., J Clin Invest. 2007 January; 117(1):143-52. Epub 2006 Dec. 21). GIP and GLP-1 are produced and secreted from intestinal K and L cells, respectively, and these peptide hormones are released in response to meals by both direct action of nutrients in the gut lumen and neural stimulation resulting from food ingestion. GIP and GLP-1 have short half-lives in human circulation due to the action of the protease dipeptidyl-peptidase IV (DPP IV), and inhibitors of this protease can lower blood glucose due to their ability to raise the levels of active forms of the incretin peptides. The glucose lowering that can be obtained with DPPIV inhibitors, however, is somewhat limited since these drugs are dependent on the endogenous release of the incretin hormones. Peptides (eg. exanatide (Byetta)) and peptide-conjugates that bind to the GIP or GLP-1 receptors but are resistant to serum protease cleavage can also lower blood glucose substantially (Gonzalez C, et al., Expert Opin Investig Drugs 2006 August; 15(8):887-95), but these incretin mimetics must be injected and tend to induce a high rate of nausea and therefore are not ideal therapies for general use in the Type II diabetic population. The clinical success of DPPIV inhibitors and incretin mimetics, though far from ideal, do point to the potential utility of compounds that increase incretin activity in the blood or directly stimulate cAMP in the beta cell. Some studies have indicated that beta cell responsiveness to GIP is diminished in Type II diabetes (Nauck M A, et al., J. Clin. Invest. 91:301-307 (1993); and Elahi D, et al., Regal. Pept. 51:63-74 (1994)). Restoration of this responsiveness (Meneilly G S, et al., Diabetes Care. 1993 January; 16(1):110-4) may be a promising way to improve beta cell function in vivo.
Since increased incretin activity has a positive effect on glucose dependent insulin secretion and perhaps other mechanisms that lead to lower blood glucose, it is also of interest to explore therapeutic approaches to increasing incretin release from intestinal K and L cells. GLP-1 secretion appears to be attenuated in Type II diabetes (Vilsboll T, et al., Diabetes 50:609-613), so improving incretin release may ameliorate this component of metabolic dysregulation. Nutrients such as glucose and fat in the gut lumen prompt incretin secretion by interaction with apical receptors (Vilsboll T, et al., Diabetes 50:609-613). GLP-1 and GIP release can also result from neural stimulation; acetylcholine and GRP can enhance incretin release in a manner perhaps analogous to the effects of these neurotransmitters on the beta cell in regard to insulin secretion (Brubaker P, Ann N Y Acad. Sci. 2006 July; 1070:10-26; and Reimann F, et al., Diabetes 2006 December; 55 (Supplement 2):S78-S85). Somatostatin, leptin and free fatty acids also appear to modulate incretin secretion (Brubaker P, Ann N Y Acad. Sci. 2006 July; 1070:10-26; and Reimann, F. et al., Diabetes. 2006 December; 55(Supplement 2):578-S85). To date, however, there does not appear to be a way to selectively impact these pathways to promote incretin secretion for therapeutic benefit. There is a need for oral drugs that stimulate incretin secretion in the treatment of diabetes.
Incretins can also increase the rate of beta cell proliferation and decrease the apoptotic rates of beta cells in animal models (Farilla L, et al., Endocrinology 2002 November; 143(11):4397-408) and human islets in vitro (Farilla L, et al., Endocrinology 2003 December; 144(12):5149-58). The net result of these changes is an increase in beta cell number and islet mass, and this should provide for increased insulin secretory capacity, which is another desired aim of anti-diabetic therapies. GLP-1 has also been shown to protect islets from the destructive effects of agents such as streptozotocin by blocking apoptosis (Li Y, et al., J Biol. Chem. 2003 Jan. 3; 278(1):471-8). Cyclin D1, a key regulator of progression through the cell cycle, is up-regulated by GLP-1, and other agents that increase cAMP and PKA activity also have a similar effect (Friedrichsen B N, et al., J Endocrinol. 2006 March; 188(3):481-92; and Kim M J, et al., J Endocrinol. 2006 March; 188(3):623-33). Increased transcription of the cyclin D1 gene occurs in response to PKA phosphorylation of CREB (cAMP-response element binding) transcription factors (Hussain M A, et al., Mol Cell Biol. 2006 October; 26(20):7747-59). There is a need for oral drugs that increase beta cell number and islet mass in the treatment of diabetes.
Beta cell cAMP levels may also be raised by inhibiting the degradation of this second messenger by phosphodiesterases to AMP (Furman B, and Pyne N, Curr Opin Investig Drugs 2006 October; 7(10):898-905). There are several different cAMP phosphodiesterases in the beta cell, and many of these have been shown to serve as a brake on glucose-dependent insulin secretion. Inhibitors of cAMP phosphodiesterases have been shown to increase insulin secretion in vitro and in vivo, including PDE1C, PDE3B, PDE10, (Han P, et al., J Biol. Chem. 1999 Aug. 6; 274(32):22337-44; Harndahl L, et al., J Biol. Chem. 2002 Oct. 4; 277(40):37446-55; Walz H A, et al., J Endocrinol. 2006 June; 189(3):629-41; Choi Y H, et al., J Clin Invest. 2006 December; 116(12):3240-51; and Cantin L D, et al., Bioorg Med Chem. Lett. 2007 May 15; 17(10):2869-73) but so far, no PDEs have been found to have the cell type selectivity necessary to avoid undesirable effects. However, this remains an area of active investigation due to the potential for amplification of the effects of incretins and other agents that stimulate adenylate cyclase.
There appear to be multiple mechanisms by which cAMP elevation in the beta cell can enhance glucose dependent insulin secretion. Classically, many of the intracellular effects of cAMP are mediated by the cAMP-dependent protein kinase (protein kinase A, PKA) (Hatakeyama H, et al., J Physiol. 2006 Jan. 15; 570(Pt 2):271-82). PKA consists of a complex of two regulatory and two catalytic domains; binding of cAMP to the catalytic domains releases the catalytic domains and results in increased protein phosphorylation activity. One of the downstream effects of this kinase activity is enhanced efficiency of insulin exocytosis (Gromada J, et al., Diabetes 1998 January; 47(1):57-65). Another cAMP binding protein is Epac, a guanine nucleotide exchange factor (GEF) (Kashima Y, et al., J Biol. Chem. 2001 Dec. 7; 276(49):46046-53. Epub 2001 Oct. 11; and Shibasaki T, et al., J Biol. Chem. 2004 Feb. 27; 279(9):7956-61), which mediates a cAMP-dependent, but PKA-independent, increase in insulin exocytosis. Epac activated by cAMP may also enhance of release of intracellular Ca++ (Holz G G, Diabetes 2004 January; 53(1):5-13). The effects of cAMP on insulin secretion are dependent on elevated glucose levels, so raising cAMP in the pancreatic beta cell is an important goal for therapeutics of Type II diabetes.
Agents that raise intracellular cAMP levels in the beta cell increase insulin secretion in a glucose dependent manner (MiuraY and Matsui H, Am. J. Physiol Endocrinol. Metab (2003) 285, E1001-E1009). One mechanism for raising cAMP is by the action of G-protein coupled cell surface receptors, which stimulate the enzyme adenylate cyclase to produce more cAMP. The GLP-1 receptor, which is the target of exanatide, is an example of such a receptor (Thorens B, et al., Diabetes (1993) 42, 1678-1682). There is a need for oral drugs that increase intracellular levels of cAMP in the treatment of diabetes.